Call Avis customer service.
If the phone rep you get does it the same way that mine did, then the rep should be able to email you a blank email for you to reply with the screenshot of your original confirmed rate attached. Then the rep will receive your email, confirm your claimed rate on your attached screenshot, and get the process rolling for Avis to redo the transaction with the price difference refunded to your method of payment. All of that can be done real time with the phone rep in one call. I got my price difference refund credited back to my credit card the next day.

I am not an Avis person. But like I said in the other thread, I used them last week for the first time in over 10 years and had a very similar experience as you did. Their system just changed my rate with no explanation and I ended up being charged $400+ for what was supposed to be total of $120. I wonder if there’s some sort of a bug in their system when you try to apply corporate discount code